The present invention relates to methods and apparatus for controlling piezoelectric vibration, and in particular for controlling piezoelectric vibration of aerosolizing devices, including, but not limited to, those that are configured to atomize liquid medicaments to be inhaled.
A wide variety of procedures have been proposed to deliver a drug to a patient. Of particular interest to the present invention are drug delivery procedures where the drug is a liquid and is dispensed in the form of fine liquid droplets for inhalation by a patient. A variety of devices have been proposed for forming the dispersion, including air jet nebulizers, ultrasonic nebulizers and metered dose inhalers (MDIs). Air jet nebulizers usually utilize a high-pressure air compressor and a baffle system that separates the small particles from the spray. Ultrasonic nebulizers generate ultrasonic waves with an oscillating piezoelectric crystal to produce liquid droplets. Examples of two such ultrasonic nebulizers are described in U.S. Pat. Nos. 5,261,601 and 4,533,082. Typical MDIs usually employ a gas propellant, such as a CFC, which carries the therapeutic substance and is sprayed into the mouth of the patient.
One exemplary atomization apparatus is described in U.S. Pat. No. 5,164,740, the complete disclosure of which is herein incorporated by reference. The atomization apparatus comprises an ultrasonic transducer and an aperture plate attached to the transducer. The aperture plate includes tapered apertures, which are employed to produce small liquid droplets. The transducer vibrates the plate at relatively high frequencies so that when the liquid is placed in contact with the rear surface of the aperture plate and the plate is vibrated, liquid droplets will be ejected through the apertures. The apparatus described in U.S. Pat. No. 5,164,740 has been instrumental in producing small liquid droplets without the need for placing a fluidic chamber in contact with the aperture plate. Thus, small volumes of liquid are delivered to the rear surface of the aperture plate.
Modified atomization apparatus are described in U.S. Pat. Nos. 5,586,550 and 5,758,637, the complete disclosures of which are herein incorporated by reference. The two references describe a liquid droplet generator, which is particularly useful in producing a high flow of droplets in a narrow size distribution. As described in U.S. Pat. No. 5,586,550, the use of a dome shaped aperture plate is advantageous in allowing more of the apertures to eject liquid droplets.
A variety of drive circuits have been proposed for vibrating piezoelectric crystals. For example, U.S. Pat. No. 4,109,174 describes a drive circuit for a piezoelectric crystal stack. The drive circuitry includes an inductor that allows the mechanically resonant structure to be electrically insensitive to a change in operating frequency. Therefore, the drive circuitry gives rise to operation at a mechanically non-resonance frequency (adjustable between 120 and 145 kHz).
U.S. Pat. No. 5,910,698 describes a methodology for driving piezoelectric vibrated mechanical structures by monitoring certain electrical characteristics, namely voltage, current and the phase difference therebetween. The electrical characteristics are measured at a first frequency where there is no significant vibration, and again at a second frequency where there is a significant vibration. The measured electrical characteristic are used to calculate the current component not relating to vibration, and this in turn is used to calculate the current component attributable to vibration. The calculated vibration current component is then adjusted so that the mechanical structure is driven at the most efficient and stable amplitude. The adjustment is achieved by comparing the calculated vibration current component with a present value, and increasing/decreasing the applied voltage when the calculated component is smaller/greater than the present value, respectively.
The present invention is related to the operation of an aerosol generator at the resonant frequency for as much of the aerosolization process as possible. Given that resonance frequency will change when a load, such as a liquid medicament, is placed on the aperture plate, there is a need to identify and change to the instantaneous resonant frequency as quickly as possible. Hence, in one aspect, the present invention is related to methods and apparatus for readily identifying and changing to the instantaneous resonant frequency of an aerosol generator.
The invention provides exemplary aerosolization devices and methods for aerosolizing liquids. In one aspect, a method is provided for aerosolizing a liquid utilizing an aerosol generator comprising a plate having a plurality of apertures and a piezoelectric element disposed to vibrate the plate. According to the method, a liquid is supplied to the plate, and the piezoelectric element is energized to vibrate the plate at an initial frequency. The amount of energy supplied to the piezoelectric element is then adjusted to vibrate the plate at a desired frequency during aerosolization of the liquid through the apertures. In this way, the liquid may be aerosolized at the highest volumetric aerosolization rate. In some embodiments, the desired frequency is the instantaneous resonant frequency of the plate, while in other embodiments, the desired frequency is an offset from the instantaneous resonant frequency. In some embodiments, the desired frequency is the anti-resonant frequency.
In one aspect, the energy adjusting step comprises detecting a first value of one or more electrical characteristics (e.g., voltage, current or phase difference between voltage and current) of the piezoelectric element at the initial frequency. The piezoelectric element is then energized to vibrate the plate at a second frequency, with the initial and second frequencies differing from each other by a predetermined amount. A second value of one or more electrical characteristics of the piezoelectric element are detected at the second frequency. The first and second values of the electrical characteristics are then compared before energizing the piezoelectric element at a third frequency. The third frequency is selected based on the comparison between the first and second values of the electrical characteristics. Hence, the change in frequency between the second and third frequencies may be different than the frequency change between the first and second frequencies. For example, the frequency change may be substantially equal to the predetermined amount if the first and second values of the electrical characteristic are substantially equal. When such values are substantially equal, the implication is that the resonant frequency is still some way off, and so there is no need to waste time by changing the frequency by anything less than the predetermined amount. Indeed, the predetermined amount could even be increased. However, if the first and second values significantly differ, the frequency change may be less than the predetermined amount, e.g., inversely proportional to the change in the measured characteristics. Such a difference in the first and second values would be indicative of proximity to the resonant frequency, and thus there is a need to change the frequency by a reduced amount in order to allow accurate determination of the instantaneous resonant frequencies.
The change in frequency may differ from the predetermined amount according to a number of algorithms. For example, as just described the change may be inversely proportional to the change in measured characteristics. Alternatively, other algorithms that may be used to find the resonance frequency include step doubling, bisection, first and second derivative tracking, and the like.
In one aspect, the method may further comprise detecting a third value of the electrical characteristic of the piezoelectric element at the third frequency, and comparing the second and third values. The piezoelectric is energized at a fourth frequency, with the third and fourth frequencies differing from each other by an amount dependent upon the comparison between the second and third values of the electrical characteristics. In other words, the procedure is iterative, with each iteration bringing the vibrational frequency closer to the instantaneous resonant frequency. In some embodiments, the iterations continue until the desired frequency is achieved. In other embodiments, a predetermined number of iterations are done.
In one aspect, the method may further comprise comparing the phase difference between voltage and current at each step. If the phase difference flips or reverses, the resonant frequency has been passed. In some embodiments, a phase locked loop circuit is used to control the frequency at which the piezoelectric plate is vibrated. As such, the frequency may be reduced and the process continued. Further, in some cases the impedance may be measured and analyzed to detect sudden changes in impedance. In this way, the end of the aerosolization event may be detected and, for example, may be used to reduce energization of the piezoelectric element.
In another embodiment, the method may further comprise energizing the piezoelectric element to vibrate the plate at or near its resonant frequency while using minimal power such that liquid is not aerosolized. In this way, the plate is maintained at or near a resonant while using minimal power. Power consumption may be increased to cause the liquid to aerosolize, and the frequency at which the plate was resonated under the minimal power situation may be used as the starting point to locate the resonant frequency under the increased power condition.
In yet another aspect, the method may comprise comparing a parameter indicative of instantaneous resonant frequency with a predetermined range of values. If the predetermined range corresponds to the limits of acceptable aerosolization, an indication of gross device failure may be given when the parameter falls outside the predetermined range of values.
In another embodiment, an aerosolizing apparatus is provided that comprises an aerosol generator having a plate with a plurality of apertures and a piezoelectric element disposed to vibrate the plate. A liquid supply is provided to supply liquid to the plate. The apparatus also comprises an energizing unit that is adapted to energize the piezoelectric element to vibrate the plate at an initial frequency and a controller for controlling the energizing unit. The controller is configured to control energization of the piezoelectric element such that the plate vibrates at its instantaneous resonant frequency during aerosolization of supplied liquid through the apertures. In some embodiments, such controllers comprise a phase locked loop circuit, while in other embodiments, operation of such controllers is primarily controlled by software.
In one aspect, the controller may comprise a detector that is configured to detect a first value of an electrical characteristic of the piezoelectric element at the initial frequency, and a second value of the electrical characteristic of the piezoelectric element at a second frequency. A comparator may be used to compare the first and second values of the electrical characteristic, and a selector may be provided to select a third frequency of energization, with the third frequency differing from the second frequency by an amount that is based upon a comparison of the first and second values of the electrical characteristic.
In other embodiments, a piezoelectric vibration control system is provided in an apparatus having a piezoelectric vibrating element and a vibratable structure to which the piezoelectric vibrating element is attached. The system comprises a detector configured to detect values of an electrical characteristic of the piezoelectric vibrating element at first and a second frequency of vibration. A comparator is configured to compare the detected values of the electrical characteristics, and a selector is configured to select a third frequency of vibration, with the second and third frequencies differing by an amount based upon a difference between the first and second frequency and a comparison of the first and second values of the electrical characteristic.
In yet other embodiments, the piezoelectric vibration control system comprises a phase locked loop circuit capable of tracking operation of the piezoelectric vibrating element and providing a frequency signal to drive a piezoelectric plate.